[boost] Re: xtime_get unsafe on Windows for IA64

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-08-26 10:26:30

Ben Hutchings wrote:
> The implementation of the boost::xtime_get() function in
> libs/thread/src/xtime.cpp includes the following code for Windows:
> 
> #if defined(BOOST_HAS_FTIME)
>         FILETIME ft;
>         ...
>         const boost::uint64_t TIMESPEC_TO_FILETIME_OFFSET =
>             ((boost::uint64_t)27111902UL << 32) +
>             (boost::uint64_t)3577643008UL;
>         xtp->sec = (int)((*(__int64*)&ft - TIMESPEC_TO_FILETIME_OFFSET)
>                          / 10000000);
>         xtp->nsec = (int)((*(__int64*)&ft - TIMESPEC_TO_FILETIME_OFFSET
>                            - ((__int64)xtp->sec * (__int64)10000000))
>                           * 100);
>         return clock_type;
> #elif ...
> 
> (This is reformatted slightly so that Outlook doesn't screw it up.)
> 
> The pointer casts are not safe because although the representations
> of FILETIME and __int64 are compatible, FILETIME is only 32-bit-
> aligned whereas __int64 must be 64-bit-aligned on IA64 (in some
> processor modes).
> 
> One can instead use union { FILETIME ft; __int64 i64; } and read out
> i64, though I'm not sure this code should be unconditionally using
> __int64 anyway.  There is a longer explanation at
> <http://weblogs.asp.net/oldnewthing/archive/2004/08/25/220195.aspx>.

How about this instead:

     const boost::uint64_t TIMESPEC_TO_FILETIME_OFFSET =
         (static_cast<boost::uint64_t>(27111902UL) << 32)
         + 3577643008UL;

     const boost::uint64_t ft64 =
         (static_cast<boost::uint64_t>(ft.dwHighDateTime) << 32)
         + ft.dwLowDateTime;

     xtp->sec = static_cast<int>(
         (ft64 - TIMESPEC_TO_FILETIME_OFFSET) / 10000000
     );

     xtp->nsec = static_cast<int>((
             ft64 - TIMESPEC_TO_FILETIME_OFFSET
             - (static_cast<boost::uint64_t>(xtp->sec) * 10000000)
         ) * 100
     );

?

Mike